I have a 2009 Mac Pro Quad Core 2.66 ghz, which I recently purchased. When running the CPU benchmarks, BOINC identifies "8" as the number of CPUs. Not a huge deal, but I think that BOINC downloads 8 tasks and attempts to do them all at once. This results, I think, in a less-than-optimum workload for my Quad Core Mac. If it would just download 4 at a time, I think it would finish the tasks much quicker.

Has anyone else experienced this problem? Is there anyway to adjust BOINC so that it sees 4 cores instead of 8?

According to Intel, your Xeon W3520 is a quad core CPU with Intel HyperThreading Technology. HyperThreading takes each physical core and makes it appear as two logical cores to get more efficient use out of the processor registers.